1. Field of the Invention
This invention relates to a printed wiring board which is capable of preventing the occurrence of lift-off and land peeling at soldered portions when a component to be mounted on the printed wiring board (hereinafter referred to as the “inserted component” is soldered by using solder, in particular lead-free solder, and an electronic apparatus such as a printer in which is installed the printed wiring board.
2. Description of the Related Art
Soldering has conventionally been carried out using eutectic lead solder (Sn—Pb: melting point 183° C.), but in recent years there have been demands for soldering to be carried out using lead-free solder due to environmental restrictions.
However, the high-temperature-type lead-free solders that are currently most commonly used are composed mainly of Sn and Ag, and have a melting point of about 220° C. If flow soldering of an inserted component is carried out using such a high-temperature-type lead-free solder, then solidification of the solder, which is accompanied by solidification shrinkage, proceeds from the vicinity of the inserted component, which has good thermal conductivity, towards the vicinity of the printed wiring board, and hence the solder joint interface at the part of the surface of the substrate on which the inserted component is mounted in particular becomes the final solidified part, resulting in lift-off and land peeling.
Moreover, when flow soldering is carried out using lead-free solder as described above, segregation of Pb contained in the surface-treated leads of the inserted component and segregation of elements (Bismuth, etc.) contained in the lead-free solder used in the flow soldering occur during the cooling process, and the physical properties of the solder changes during the cooling process. As a result, there is a problem that the occurrence of lift-off and land peeling is increased, and in the worst cases the land peeling is accompanied by breakage (i.e., electrical disconnection) of the pattern connected to the lands.
To resolve such a problem, Japanese Laid-open Patent Publication (Kokai) No. 11-354919 discloses a method of improving countermeasures in the cooling step of the soldering process. However, there is a problem that a coolant must be used during the cooling, and hence the soldering cost rises.
FIG. 5 is a sectional view of a conventional example of a printed wiring board in which lift-off has occurred, and FIG. 6 is a sectional view of a conventional example of a printed wiring board in which land peeling has occurred.
In FIGS. 5 and 6, reference numeral 1 designates a substrate of the printed wiring board. A plurality of through holes 5 are formed in the substrate 1, and a land 6 is formed over an inner peripheral surface of each through hole 5 and opposite end surface parts of the substrate 1 formed with the through hole 5. Reference numeral 3 designates an inserted component such as an electronic component having inserted component leads 2. Each inserted component lead 2 is inserted into one of the through holes 5, and soldered to the substrate 1 with lead-free solder by flow soldering. Reference numeral 8 designates a solder resist.
In the case of a conventional printed wiring board on which lead-free soldering is carried out as described above, during the flow soldering the lead-free solder rises through each through hole 5 in a wet state, thus forming a fillet 7 between a mounting surface part of the inserted component 3 at which it is mounted onto the substrate 1 and the land 6. As a result, lift-off in which the fillet 7 between the mounting surface part of the inserted component 3 and the land 6 does not join to the land 6 may occur as shown in FIG. 5, or the land 6 may peel off along with the fillet 7 as shown in FIG. 6.